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The Red Blood Corpuscle

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THE RED BLOOD CORPUSCLE The primary object of the red blood ,corpuscle is that of an oxy gen transporter. Its efficiency as such depends upon the quantity of haemoglobin it contains. The average number of red corpuscles in man is 5,000,000,000 per cubic centimetre of blood. The aver age weight of haemoglobin in the same is 0.13 gram and the average amount of oxygen which these will transport is •185cu.cm. This is more than so times the quantity which could be dissolved in a cubic centimetre of blood in its passage through the lung, in the absence of haemoglobin. The intensive oxidation in the tissues of warm-blooded animals has only been made possible by the existence and utilization of haemoglobin.

The number of red blood corpuscles per cubic millimetre of blood varies in different animals thus :— Even in the same person the number differs considerably at ent times; factors which induce rapid sweating cause an increase; the number increases also at high altitudes. (See ANOXAEMIA.) The size and shape also vary. In the mammals the cells are round, except in the camel, which has oval corpuscles. In the lower vertebrates they are oval and nucleated. Thus:— Diameter of red blood corpuscles in thousandths of a mm. (A) The above figures are only approximate, for as stated below a single corpuscle alters somewhat in size from time to time, and in the same individual the size of the red blood corpuscles may vary somewhat according to the conditions of stress under which they are produced.

Specific Gravity.

The specific gravity of blood depends principally upon the quantity of haemoglobin present ; normally it is between 1,055 and 1,060, distilled water being 1,000. Since the methods for the measurement of haemoglobin have been improved, less account has been taken of specific gravity. The red blood corpuscle in man consists of a minute disc-shaped body having a diameter of 7µ (rµ =1.00o mm.) and a thickness of about 1 µ. It is very elastic and in the small vessels may take up almost any shape, as has recently been shown by Krogh. It is not, how ever, amoeboid.

In composition

the red corpuscle consists of 6o% water and 40% solids, of which 90% is the red pigment haemoglobin. In the larger vessels the red corpuscles tend to form "rouleaux" an arrangement which is likely to occur in the case of any series of discs with fatty surfaces, floating in an aqueous fluid. Although in mammals they are normally nonnucleated, under abnormal con ditions such as anaemia nucleated corpuscles appear in the cir culating blood.

The intimate structure of red blood corpuscles has been a matter of much controversy, some authors regarding them simply as bags of fluid and others as consisting of a sponge work or stroma, which is more close at the outside than elsewhere. In the absence of ascertained knowledge of the correctness of either view it may be said that discussions on the chemistry and physics of the red corpuscles are usually carried out on the basis of the former, the corpuscle being regarded as material surrounded by a membrane possessing certain definite chemical and physical char acteristics. The membrane is permeable, for instance, to water, and to acid radicles, but less so to sodium, potassium, etc. These facts form the basis of complicated readjustments of equilibria between the constituents of the corpuscles and those of the plasma, when the latter are altered. Among the materials which may pass through the corpuscle membrane is water, therefore the corpuscle may swell and shrink. Normally it swells if the medium in which it is placed is made more acid.

Haemolysis.

The passing of water into the corpuscle may take place on such a scale as ultimately to burst the membrane, the solution of haemoglobin previously enclosed escapes into the surrounding fluid, and the structures which contained the haemo globin are left as "shadows." This process is known as "laking" or haemolysis which may be induced by the addition of distilled water to blood. The corpuscles may be ruptured in other ways, by freezing and thawing, by the addition of saponin, or bile salts. More remarkable is the fact that if the blood corpuscles of one species are injected into the vessels of another they are destroyed, their haemoglobin being liberated. This phenomenon is not due to osmotic or similar changes, but to some specific sub stance in the plasma which attacks the foreign corpuscles. That substance is called a haemolysin. It is one of several substances on which the immunity of the body from disease depends. (See IMMUNITY.) Fragility of Red Blood Corpuscles.—All corpuscles are not disrupted with equal ease by haemolytic agents. Thus whilst all or nearly all corpuscles are disrupted by distilled water, salt solu tions of less saline concentration than plasma will disrupt some and not others: the "fragility" of the corpuscles differs. Thus in the case of a cat : Strength of salt solution . •64 .6o .56 •48 .44 .40 Percentage of corpuscles haemolysed . . . •0 8 45 80 88 93 99 The resistance of the red cells of some animals is greater than that of others. If different animals are graded as regards the resistance of their red cells to saponin the following order would be ob served: lamb, goat, ox, cat, grey mouse, pig, grey rat, dog, white rat, rabbit and guinea-pig. The resistance of their red cells to hypotonic salt solutions is, on the contrary, in precisely the opposite order.

Significance of Red Corpuscles.

This is both physical and chemical.

Physical: (I) In the fine vessels the diameter of which is not larger or not much larger than a red corpuscle, the packing of the haemoglobin in the corpuscle ensures that the propulsion of the whole of the haemoglobin along the vessel, were it simply dis solved, would result in a flow in the vessel of the streamline character. Movement would be confined principally to the central core of fluid. (2) The vessel walls are so delicate that haemo globin diffuses through them. Were they made less diffusible so as to retain the haemoglobin, they would offer a greater resistance to other substances which require to traverse them quickly.

Chemical : ) The exchange of oxygen between blood and haemoglobin should take place most efficiently in a medium slightly less alkaline than the blood plasma; this medium is sup plied in the corpuscle. (2) The haemoglobin of many animals crystallizes easily ; the corpuscle, by what means we do not know, holds it in solution—a condition necessary to its function.

The origin of red corpuscles in the embryo is in the forming blood vessels. These are laid down as ropes of tissue which ulti mately become differentiated, the outer cells forming the vessel wall, the inner acquiring haemoglobin and becoming the nucleated ancestors of the red blood corpuscles. About half way through embryonic life in the mammal these nucleated ancestors disappear from the general circulation and are found only in the spleen, the liver and the bone marrow, and at a later stage the bone marrow is the sole normal breeding ground for red blood corpuscles. The activity of the bone marrow during life is regulated to some de gree by circumstances; any exaggerated destruction of corpuscles such as bleeding, or the injection into the veins of such drugs as pyrogallic acid, will awake the bone marrow to greater production. So also will conditions which reduce the efficient functional ac tivity of the corpuscles, such as exposure to low oxygen pres sures. On the other hand, the normal activity of the bone marrow may become impaired, resulting in certain grave kinds of anaemia.

The longevity of a red corpuscle has been placed at 30-70 days. The calculation does not rest on any very reliable basis. Probably it is safe to say that the lifetime of the red corpuscle is to be measured in months, being longer than weeks and shorter than years. The ultimate fate of the pigment of red corpuscles is for the most part to have its iron detached in the liver and for the rest of the haematin moiety to be secreted as bile pigment. The debris of broken-down corpuscles is found in the spleen.

corpuscles, haemoglobin, water, animals and vessels